High voltage generator for electrostatic painting equipment

ABSTRACT

The invention concerns a generator, housed inside the equipment, such as a sprayer, a high voltage cascade including a voltage-gain transformer ( 2 ) and a voltage multiplier ( 3 ) arranged at the output of the transformer ( 2 ). Said transformer ( 2 ) is a double or multiple transformer, consisting of at least two elementary transformers ( 2 A,  2 B), whereof the respective primary windings ( 8 A,  8 B) are mounted electrically in parallel, and whereof the respective secondary windings ( 9 A,  9 B) are mounted electrically in series, said arrangement enabling to reduce the diameter of the cascade, to facilitate its incorporation in the sprayer or other equipment. The invention is applicable to painting robots for the automotive industry.

[0001] The present invention relates, in a general manner, toelectrostatic painting equipment, such as sprayers used on paintingrobots, in the automotive industry. More particularly, this inventionconcerns an assembly for generating high electrical voltage, capable ofgenerating a high voltage (typically of several tens of kilovolts)applied to the electrode of such electrostatic painting equipment, withthe aim of electrically charging the paint, so as to improve theefficiency of deposition of this painting equipment and its quality ofpaint application.

[0002] Such devices are known, for example, through the documents U.S.Pat. No. 3,608,823 A, U.S. Pat. No. 3,731,145 A and U.S. Pat. No.5,067,434 A.

[0003] These devices generally consist of the combination of anelectronic control unit, generating an AC voltage of variable amplitudeand variable frequency, the so-called “low voltage”, and of a “cascade”comprising in particular a transformer and a voltage multiplier.

[0004] The electronic control unit delivers, in general, a substantiallysinusoidal, low AC voltage whose frequency is of the order of a few tensof kilohertz (in particular lying between 20 and 50 kHz) and whoseamplitude (peak-to-peak) is of the order of a few tens of volts.

[0005] The transformer customarily has a turns ratio of around 100, andits output voltage is of the order of 10 000 volts (peak-to-peak). Themultiplier consists of rectifier stages each comprising a capacitor anda diode, two consecutive stages being mounted in series in such a way asto double the peak voltage emanating from the transformer.

[0006] The trend in the art of paint sprayers is moving toward, on theone hand, greatly reducing their bulkiness, and in particular theiroutside diameter, so as to reduce the soiling caused by the excess paintsprayed in the painting booth.

[0007] However, on the other hand, the standards in force, in Europe andin the U.S.A., are currently making it necessary to integrate the highvoltage “cascade” into the sprayers, so as to minimize the storedelectrical energy which is liable to be discharged in the event ofsparkover. More precisely, according to the currently applicablestandards, the energy liberated during a sparkover must be less than0.24 millijoules, for solvent-based liquid paints, and be less than 5millijoules for powder paint, so as not to cause ignition. Hence, theenergy in the high voltage “cascade”, as well as in the sprayer itself,must be minimized.

[0008] These requirements have already led to the high voltage,“cascade” being placed inside the sprayer, given that, if this “cascade”is situated outside, it has to be linked to the sprayer by a highvoltage cable, which is by nature capacitive, and which would store uptoo much energy.

[0009] The housing of the high voltage “cascades” inside the sprayersleads to the reducing of their dimensions, and in particular of theircross section. In order to maintain the appropriate insulationdistances, for high voltages of the order of 100 kV, between theelectrode of the sprayer and the “low voltage” side of the “cascade”, aminimum distance, of the order of some twenty centimeters, is necessary.The “cascade” must therefore be designed in such a way as to extendlengthwise along the sprayer. Stated otherwise, for a “cascade” of givenvolume, it is better for it to possess a greater length and a smallercross section.

[0010] This requirement results also from the diameters of the sprayersbeing ever smaller, as already indicated above, these sprayers stillhaving to house other components: valves, various ducts (air, solvent,paint), motor, turbine, device for measuring turbine speed (for exampleby optical fiber).

[0011] The total length of the sprayers must however remain reasonable,since this length has an influence on the total width of the paintingbooths.

[0012] The cluttering of the voltage multiplier with a high voltage“cascade”, consisting of stages with capacitor and diode, may be reducedto the maximum extent by placing the diodes as near as possible to thecapacitors, and by reducing the diameters of the capacitors.

[0013] However, in dealing with capacitors whose dielectric consists ofa ceramic disk, the value of the capacitance of these capacitors isgiven by the formula: C=∈×d/s, where:

[0014] ∈ is the dielectric constant of the ceramic,

[0015] d is the height of the ceramic disk,

[0016] s is the cross section of the ceramic disk.

[0017] By applying this formula, it is noted that, for a givendielectric constant of the ceramic, a decrease in the cross section ofthe ceramic disk leads to an increase in the value of the capacitance,and hence of the stored energy, this being detrimental to safety asalready discussed above. Moreover, it is necessary to use, for thecapacitors, a ceramic whose dielectric constant is as stable as possibleas a function of temperature, thus leaving a limited choice ofmaterials, and hence of the factor ∈.

[0018] Additionally, the voltage multiplier is not, currently, the partof the high voltage “cascade” having the largest cross section, thetransformer customarily possessing a greater cross section. Thus, solelyreducing the diameter of the capacitors of the voltage multiplier is notsufficient to solve the problem posed here, and it would also benecessary to intervene on the transformer.

[0019] The transformer remains, however, the most difficult part toimplement, and in particular to miniaturize, while retaining the powerrequired here, of the order of 15 VA, with a turns ratio of the order of100.

[0020] The primary of the transformer is determined in such a way as tohave a minimum magnetic induction of the order of 0.1 to 0.2 teslars, soas to minimize the iron losses, given by the formula: Piron(B)−B^((2+x)) with: 0≦x≦1. This makes it necessary to have a largenumber N1 of turns at the primary of the transformer, by applying theformula: B˜1/N1. Consequently, and in view of the desired turns ratio,the secondary of the transformer must comprise several thousand turns.

[0021] The turns of the primary are, in general, situated in the heartof the transformer, while the turns of the secondary are arranged asseveral layers, toward the outside. This involves large totaltransformer cross sections.

[0022] It is also advisable to minimize the overall losses in thetransformer, which losses are made up in particular of the copper lossesand of the iron losses.

[0023] The copper losses depend on the resistance of the primary windingand on the resistance of the secondary winding. Each of theseresistances depends on the total length of the winding and on the crosssection of the wire which constitutes this winding. Thus, a smaller wirecross section entails a larger resistance, for equal length, andconsequently higher copper losses. Likewise, an increase in the numberof turns increases the total length of the winding, hence the copperlosses. Simply decreasing the wire cross section, or increasing thenumber of turns, do not therefore constitute satisfactory solutions perse.

[0024] As far as the iron losses are concerned, these depend on themagnetic induction, as already explained above, and also on thefrequency and the nature of the magnetic material used to conduct themagnetic flux, as well as the overall volume of this material. In orderto operate at a frequency of a few tens of kilohertz, one uses ferrites,here chosen from among the various kinds and forms of ferritesconventionally used in the field of electronics, to conduct the magneticflux. In this regard, likewise, the current solutions are notsatisfactory; in particular, a typical known implementation consists ofa transformer wound on a cylindrical ferrite core, which does not closeup the magnetic flux, so that the output voltage decreases as thecurrent increases; the efficiency of deposition of the paint, whichdepends on this output voltage, also diminishes with increasing current.

[0025] Similar problems, in particular of a dimensional nature, alsoarise in the case of handheld guns for paint spraying.

[0026] The present invention aims to solve all the problems set forthabove, by proposing a high voltage “cascade” implementation which isimproved also in its “transformer” part, in such a way as to allow anappreciable reduction in diameter of the “cascade”, facilitating itsincorporation into the sprayer or other equipment, while retaining thepower required and improving the operating conditions of the assembly.

[0027] For this purpose, the subject of the invention is essentially ahigh voltage-generator for electrostatic painting equipment, saidgenerator comprising in particular, housed inside the equipment, a highvoltage “cascade” comprising on the one hand a voltage boostertransformer, and on the other hand a voltage multiplier placed at theoutput of the transformer, this generator being characterized in thatsaid transformer consists of a double or multiple transformer, made upof at least two elementary transformers, whose respective primarywindings are mounted electrically in parallel, and whose respectivesecondary windings are mounted electrically in series, the seriesmounting of the secondary windings providing an output voltage, the sumof the voltages across the terminals of each of the secondary windings,and which is the input voltage of the voltage multiplier.

[0028] For example, to implement the equivalent of a single transformerwhose turns ratio is equal to 100, the invention can be employed bycombining two elementary transformers, each of ratio equal to 50, therespective primaries of the two elementary transformers being mounted inparallel and energized thus, both, by the low voltage, while therespective secondaries of these two elementary transformers, mounted inseries, provide an appropriate output voltage (double the voltage acrossthe terminals of each secondary) across the terminals of this seriesmounting, said output voltage being the input voltage of the voltagemultiplier.

[0029] The generator, which is the subject of the invention, is thuscharacterized by the particular combination of two or more elementarytransformers in the high voltage “cascade”. A preferred form ofimplementation of the invention provides for these two or moreelementary transformers to be arranged coaxially, following one another,in such a way that the maximum diameter or the maximum cross section ofthe “cascade” is not greater than the diameter or than the cross sectionof each elementary transformer.

[0030] Advantageously, each elementary transformer is a transformer ofcylindrical or substantially cylindrical outside shape, with apot-shaped ferrite part, ensuring magnetic flux closure.

[0031] Thus, for each of the elementary transformers, the number ofturns of the secondary is divided by the number of elementarytransformers (by comparison with the current solution with a singletransformer). Certainly, the invention leads to an increase in thenumber of primaries but, all the primaries being mounted electrically inparallel, the electric current flowing through them is divided, and itis thus possible to implement them with a smaller wire cross section.Moreover, in an elementary transformer of smaller overall cross section,the turns of the primary are less “buried” (situated at depth) and canbenefit from more satisfactory thermal exchange. Additionally, thepotential difference between turns, and between layers of turns, at thesecondary, is smaller since the overall potential between the input andthe output of the secondary winding of each elementary transformer isdivided by the number of elementary transformers, equal for example to 2or 3. This makes it possible to reduce the tolerances in respect of theleakage lines of the windings, and to fill in the window of the ferritepart in an optimal manner.

[0032] All these elements make it possible to reduce the cross sectionof the elementary transformers, hence the cross section of the “cascade”itself, insofar as this cross section is determined essentially by that,of the “transformer” part, and as the elementary transformers, forexample two or three in number, are preferably arranged coaxially.

[0033] According to a complementary characteristic of the invention, inorder that the reduction in the dimensions of the high voltage “cascade”is not accompanied by a disadvantageous decrease in the insulationdistance, provision is made for the “high voltage” side connection ofthe generator to be effected through an arrangement of baffles, makingit possible to lengthen the insulation distance of the high voltage withrespect to ground, without however increasing the geometrical length ofthe “cascade” and without requiring any additional item.

[0034] Overall, one thus obtains the high voltage “cascade” that can bereadily incorporated into a handheld paint gun or sprayer, even with areduced diameter, without this requiring an undesirable lengthening ofthe sprayer or gun.

[0035] The invention will be better understood with the aid of thedescription which follows, with reference to the appended schematicdrawing representing, by way of examples, a few embodiments of this highvoltage generator for electrostatic painting equipment:

[0036]FIG. 1 represents, in perspective, a high voltage “cascade” ofsuch a generator, with its various constituent elements;

[0037]FIG. 2 is a circuit diagram of the transformer of the “cascade” ofFIG. 1;

[0038]FIG. 3 represents in perspective the same “cascade” molded in aninsulating resin;

[0039]FIG. 4 is a perspective view, simplified, of a paint sprayer intowhich such a “cascade” is integrated;

[0040]FIG. 5 illustrates, in circuit diagram form, a variant of thetransformer.

[0041]FIG. 1 represents a high voltage “cascade”, with its constituents,arranged in line along one and the same central axis A, which aresuccessively: a “transformer” part 2, a voltage multiplier 3, a currentlimiting resistor 4, and a high voltage contact 5, all these componentsbeing connected together, mechanically and electrically. The assembly isenergized electrically, on the transformer 2 side, with an AC voltageprovided by an electronic control unit (not represented), this “lowvoltage” being brought to input terminals 6 and 7.

[0042] According to the invention, and in the example illustrated herein FIGS. 1 and 2, the voltage booster transformer 2 is a doubletransformer, composed of two elementary transformers 2A and 2B, ofidentical structure, stationed side by side.

[0043] In detail, and as shown more particularly in FIG. 2, the firstelementary transformer 2A possesses a primary winding 8A and a secondarywinding 9A; the two windings 8A and 9A are coiled around the centralcore of a ferrite pot 10A, for example of the so-called “RM” type, whichallows the magnetic flux to close up. Similarly, the second elementarytransformer 2B possesses a primary winding 8B and a secondary winding9B; the two windings 8B and 9B are coiled around the central core of aferrite pot 10B, which allows the magnetic flux to close up.

[0044] From the two input terminals 6 and 7, the respective primarywindings 8A and 8B of the two elementary transformers 2A and 2B aremounted electrically in parallel, so that each primary winding 8A or 8Bis energized under the same input voltage Ve, but is traversed by acurrent of strength i/2 equal to half the strength i of the currentdelivered by the control electronics.

[0045] On the other hand, the respective secondary windings 9A and 9B ofthe two elementary transformers 2A and 2B are mounted electrically inseries, so that the voltage across the terminals of each secondarywinding 9A or 9B is equal to half, i.e. Vs/2, the output voltage Vs ofthe transformer part 2.

[0046] This transformer part 2 possesses, typically, a turns ratio equalto 100, that is to say its output voltage Vs is equal to 100 times itsinput voltage Ve. To achieve this result, it is advisable for eachelementary transformer 2A or 2B to possess a turns ratio equal to 50,this determining the ratio of the numbers of turns of the primary coils8A, 8B and secondary coils 9A, 9B of these elementary transformers 2Aand 2B.

[0047] The output voltage Vs of the transformer part 2 is againmultiplied, by the desired factor equal, for example to 20, in thevoltage multiplier 3 which, in a manner known per se, comprises a seriesof rectifier stages with capacitors 11 and diodes 12, the voltage Vsconstituting the input voltage of this voltage multiplier 3.

[0048] As shown in FIG. 3, the assembly of the previously describedcomponents is advantageously sunk into an embedding material 13, moreparticularly into an insulating resin. This embedding material 13 forms,in front of the “cascade”, that is to say in the region of the highvoltage contact 5, baffles 14 which make it possible to lengthen theinsulation distance of the high voltage, without thereby increasing thetotal constructional length (along the axis A) of said “cascade”.

[0049] The high voltage “cascade”, implemented as has just beendescribed, can be incorporated into an electrostatic paint sprayer 15,in the manner illustrated very schematically in FIG. 4, where this“cascade” is indicated, in its entirety, by the label 16. The crosssection of the two elementary transformers 2A and 2B determines themaximum cross section of the “cascade” 16, it being possible here forthis cross section to be maintained at a relatively small value. Thesprayer 15 is carried by a painting robot, not represented.

[0050] The number of elementary transformers, which make up the“transformer” part 2, may be greater than 2, this allowing an additionalreduction in the maximum cross section of the “cascade”. Thus, FIG. 5illustrates a variant, in which this part 2 is made up of threeelementary transformers 2A, 2B and 2C. The respective primary windings8A, 8B and 8C of the three elementary transformers 2A, 2B and 2C aremounted in parallel, hence each energized under the input voltage Ve.The respective secondary windings 9A, 9B and 9C of these threeelementary transformers 2A, 2B and 2C are mounted in series, the voltageacross the terminals of each of them being equal to a third (Vs/3) ofthe output voltage Vs.

[0051] The number of elementary transformers of the “cascade”, or theirmultiplication ratio, or else the number of stages of the voltagemultiplier, may naturally be modified without departing from the scopeof the invention. With the same idea in mind, the invention isapplicable not only to paint sprayers carried by painting robots, butalso to handheld paint guns, and to other similar equipment. Finally,the high voltage generator, which is the subject of the presentinvention, is equally applicable to equipment for powder paint as toequipment for liquid paint.

1. A high voltage generator for electrostatic painting equipment (15),said generator comprising in particular, housed inside the equipment(15), a high voltage “cascade” (16) comprising on the one hand a voltagebooster transformer (2), and on the other hand a voltage multiplier (3)placed at the output of the transformer (2), characterized in that saidtransformer (2) consists of a double or multiple transformer, made up ofat least two elementary transformers (2 a, 2B, 2C), whose respectiveprimary windings (8A, 8B, 8C) are mounted electrically in parallel, andwhose respective secondary windings (9A, 9B, 9C) are mountedelectrically in series, the series mounting of the secondary windings(9A, 9B, 9C) providing an output voltage (Vs), the sum of the voltages(Vs/N) across the terminals of each of the secondary windings, and whichis the input voltage of the voltage multiplier (3).
 2. The high voltagegenerator as claimed in claim 1, characterized in that the two or moreelementary transformers (2A, 2B, 2C) are arranged coaxially, followingone another, in such a way that the maximum diameter or the maximumcross section of the “cascade” (16) is not greater than the diameter orthan the cross section of each elementary transformer (2A, 2B, 2C). 3.The high voltage generator as claimed in claim 1 or 2, characterized inthat each elementary transformer (2A, 2B, 2C) is a transformer ofcylindrical or substantially cylindrical outside shape, with apot-shaped ferrite part (10A, 10B), ensuring magnetic flux closure. 4.The high voltage generator as claimed in any one of claims 1 to 3,characterized in that the “high voltage” side connection of thegenerator is effected through an arrangement of baffles (14), making itpossible to lengthen the insulation distance of the high voltage withrespect to ground.
 5. The high voltage generator as claimed in any oneof claims 1 to 4, characterized in that the painting equipment is anelectrostatic paint sprayer (15) carried by a painting robot.